1. Field of the Invention
The field of the invention is that of ferrite materials used at high and very high frequencies (1 megahertz to 100 megahertz) and especially materials with low magnetic losses and low manufacturing temperatures.
Materials of this kind are in very great demand for the making of small-sized inductors used in filtering devices that work between 1 megahertz and 100 megahertz and more generally for the low-temperature manufacture of magnetic cores having low losses.
2. Description of the Prior Art
At present, the development of electronic equipment, in civilian as well as military applications, is linked to the miniaturization of the active and passive components used. Among these components, the most bulky ones are the passive components which fulfil the functions of resistors, capacitors and more particularly inductors.
In applications using so-called "low level" inductors implementing low electrical power values, the losses of the component are represented by a Q factor where Q=L.omega./R, with L as the series inductance, .omega. as the pulsation and R as the series resistance. The greater the coefficient Q, the higher the quality of the corresponding electrical circuit.
Ferrites with high Q factor values (of over 100) in high-frequency applications (of over one megahertz) are generally composite nickel-zinc ferrites. They are used as magnetic cores of a variety of shapes (toroid, pot-shaped, rod-shaped, etc.) enabling the making of wound inductors, the winding portion being made by means of enamelled copper wire. However, these materials have high manufacturing temperatures. Their firing temperature (firing being the heat treatment designed to form the required crystalline phase) is in the region of 1000.degree. C. to 1100.degree. C. Their sintering temperature (sintering being the heat treatment aimed at the complete formation of the crystal phase and the densification of this crystal phase) is in the region of 1200.degree. C. to 1350.degree. C.
Materials of this kind therefore cannot be used to make small-sized inductors, also called micro-inductors, into which the magnetic core and winding parts are integrated by co-sintering the ferrite of the magnetic core with the metal of the winding (which is conventionally silver).
Indeed, at present, micro-inductors that work in particular between 1 megahertz and 100 megahertz are made out of ferrites with relatively low sintering temperatures (below 1000.degree. C.) which are indispensable to the prevention of the chemical reaction of the metal and especially silver. This reaction would have the consequence of causing the deterioration of the electrical properties of the metal and of the electromagnetic properties of the ferrite. The materials used for these applications form part of the family of nickel-zinc-copper ferrites which have electromagnetic properties close to those of nickel-zinc ferrites. The addition of copper in oxide form enables the sintering of these materials at temperatures of 900.degree. C. to 1000.degree. C. However, it lowers the Q factor values which drop 20 to 60 for the micro-inductors.